Land-based gas turbine engines, which are primarily used for generating electricity, include a combustion system that mixes fuel with compressed air from the engine compressor and contains the reaction that generates hot combustion gases to drive a turbine. The combustion system injects a fuel, typically natural gas or a liquid fuel, to mix with the compressed air. Combustion systems which inject either fuel type are typically referred to as dual fuel combustors. This type of combustion system offers flexibility to the engine operator with regard to which fuel to use, depending on fuel availability, fuel costs, and level of emissions allowed. While natural gas fired gas turbine engines have become increasingly popular due to lower levels of NOx emissions produced, not all regions of the world in which gas turbine engines operate are regulated by emissions nor is natural gas a desired fuel choice for economic reasons.
While dual fuel combustion systems provide the flexibility to operate on different fuel types, they have exhibited some shortcomings, especially during the liquid fuel operation. More specifically, the combustor hardware surrounding the liquid fuel nozzle has been known to exhibit carbon buildup over a period of time. Build up of carbon has resulted in poor performance and damage to the fuel nozzles and combustion liner components requiring premature repair and replacement. Often times, engine operators have been required to limit the amount of time operating on liquid fuel in order to limit the amount of carbon buildup.
A specific example of a fuel nozzle known to exhibit carbon buildup is shown in FIG. 1. Fuel nozzle 10 includes gas tip 11 and liquid nozzle 12, which includes a plurality of concentric tubes 13, 14, and 15. Inner tube 13 contains a liquid fuel such as oil, while middle tube 14 contains water, and outer tube 15 contains air. Surrounding liquid nozzle 12 is gas tip 11 that injects a gaseous fuel through injection holes 17 to mix with the surrounding air in mixing tube 16. Whether fuel nozzle 10 is operating on liquid fuel or gaseous fuel, the fluids mix in mixing tube 16. It is during the liquid fuel operation that this prior art design has exhibited carbon buildup along the tip region of fuel nozzle 10 and along mixing tube 16. The carbon buildup is a result of recirculation zones within mixing tube 16, particularly along the interface between fuel nozzle 10 and mixing tube 16, such that liquid fuel droplets are redirected to impinge on the tip of fuel nozzle 10 and along mixing tube 16, adhering to the surface and forming carbon deposits. Over time, the carbon deposits build-up to a level that impairs fuel nozzle and combustor performance, requiring repair and replacement.
Referring to FIGS. 2 and 3, a second prior art fuel nozzle 30 is shown in detail and is the subject of U.S. Pat. No. 5,833,141. In order to prevent the carbon build up exhibited in fuel nozzle 10, fuel nozzle 30 was positioned such that the liquid nozzle portion extended the full length of the mixing tube and is combined with an additional outer swirler 31 and therefore reduced the possibility of recirculation of liquid fuel droplets onto the fuel nozzle or mixing tube 36. While this design has proven to reduce the amount of carbon buildup, it requires modifications to the gas/air swirler of the prior art fuel nozzle 10, including extending the swirler channel and incorporating an additional outer swirler.